Keeping the Chassis Battery Charged with Solar

We’ve done a lot of work on the electrical systems in our camper-van, Lance. Our house battery is a large lithium-ion battery, and we’ve got 300 watts of solar, an inverter/charger, and a supplemental alternator to keep it charged up. With those working for us, on most days, I don’t even think about the state of charge of our house battery. But as much as all of that does for the house battery… it does nothing to keep the chassis battery charged up. That’s the battery that starts the engine!

During times when we’ve stored Lance, I’ve noticed the chassis battery being drawn down by parasitic loads. This is largely due to accessories that we’ve installed, so I know what’s causing it. But I still needed a way to overcome those loads and keep the chassis battery charged up when we have the rig stored. That led to what you see in this video:

What I show in the video is basically a smaller version of the solar power system that’s installed on our vehicle for the house battery. All of the components come from Zamp Solar, so it truly is plug and play. Even the mechanically non-inclined shouldn’t have any trouble setting this up. The components I show in the video are listed out here.

Zamp 20 Watt Solar Panel - This is a basic, small solar panel that is more than enough to overcome any parasitic draw that we have on our chassis battery. Depending on what accessories you have installed onto your chassis, you could do this with a larger or smaller panel. At 20 watts, the size is quite manageable. Since the panel is larger than we need, I don't worry too much about pointing it, or shade, or anything like that. Typically, I'll just set it out and forget about it.

Zamp 8 Amp Plug-N-Play Solar Regulator - While the solar charge controller installed on our rig can handle 40 amps, this is a smaller version that can handle 8 amps. It's self contained, and there's nothing to set up and nothing to program. (I guess that's why they call it "Plug-N-Play".) All you have to make sure to do is plug the panel and charge cables into the appropriate, respective places on the controller. (They're labeled...)

Zamp Solar BDCBC Clamp Connector - You need a way to get the output from the solar regulator to your battery, and these battery clamps are the easiest way to do it. You simply connect them to your vehicle as you would if you were jump-starting. On our ProMaster, that's really easy - but you need to follow the proper procedure for your rig - whatever that may be. The other end plugs right into the charge controller.

Zamp Solar BDCEXT15 Extension Cord - I didn't show this in the video, but if you need a way to get the solar panel and/or charge controller a bit further away from your vehicle, this extension cord will allow you to do that. You might need it if your RV is stored under a carport and you want to put the solar panel on the roof, for example. So depending on your situation, you may or may not need this.

And those three (or four) things are basically it. If you’re looking for a way to keep your RV’s chassis battery charged up and ready to go, this, or something like it, is the ticket. And you don’t have to limit yourself to RVs here – this system would work with any lead-acid battery.

James is a former rocket scientist, a USA Cycling coach, and lifelong fitness buff. When he's not driving the RV, or modifying the RV (or - that one time - doing both at once), you can find him racing bicycles, or building furniture, or making music. In his spare time, he works for a large IT company.

34 thoughts on “Keeping the Chassis Battery Charged with Solar”

I own a 2007 Monaco Diplomat with an Aladin panel with memory in it. I just lost my storage spot with shore power so I need solar charger with a battery “tender” in it. I have 2 new group 31 batteries on board for the chassis batteries. I understand the Aladin draws .5 amp to maintain the memory. I don’t dry camp and I just want to maintain the 2 chassis batteries while in storage (a week to a month). I am in southern California so cold weather is not an issue. I don’t know what size solar panel to buy.

I don’t know what other loads you have going, but let’s assume it’s just the Aladin panel for now. .5 amps x 12 volts = 6 watts. 6 Watts x 24 Hours = 144 Watt Hours Given that, I’d go with a 100 watt panel. That will allow for clouds, rain, shorter days, etc. But that might change if you have more loads running than just the .5 amp.

I am getting mixed advice on connecting to the posts under the hood. The convenience is unbeatable, and I don’t like working around the battery. But I suppose a direct connection to the battery in the floor would be better? Or does it really matter? Thanks! Always love the information you provide.

Doing a similar mod… just go directly to the battery. If you are ever trying to diagnose an automotive electrical issue, one of the first things people do is run wires directly to the battery to rule out the possibility of a short somewhere else along the circuit. I can’t think of a single good reason to connect it to the jump-start terminals under the hood – besides that it *maybe* would save you a couple minutes on the install.

Is it possible to split the power line from existing solar to be input to 2 separate controllers, one for the chassis and the larger one for the coach? Do you need some sort of diode between them or ??

I apologize but this comment has nothing to do with your topic….I saw your little yellow tow utility trailor in this video parked in the driveway….With occc being a big determining factor in RV choices….can I request that you make a video on your trailor specs model and maufacturer…..I love the size and I know your choice of this was well thought out……Thx again and thx for all the great content…..

A dual voltage sensing relay for alternator charging will connect the two when either rises above the connection setpoint. Usually something like 13.4v.

A single voltage sensing relay will keep the two connected: * the ignition is turned off when * solar is keeping the house bank (and therefore the still-connected chassis battery) above the setpoint.

One might move the power wire for a regular solenoid/relay from the ignition to a voltage sensor that powers the solenoid when Vbank is in a sweet spot” for the chassis battery, maybe 12.7v – 13.8v. An $11 board will do the trick (amazon item B01GJM04GC). It’d be automatic and hidden from sight.

I already had the VSR installed so I put one of those on the VSR to disable it when house bank voltage rises above 13.7v. The VSR itself cuts off at 13.4v so the board’s LVD is moot.

Currently I have AGM for house and chassis, with a fair sized (560W) solar array, and about 400AH house AGM bank. I’m using a VSR (Blue sea ML-ACR) to couple the house and chassis batteries, so the solar, or driving/alternator, or shore power can charge the battery banks, pretty much transparently. I have the solar and shore set up with a proper AGM profile, and the alternator will get things close.

I’m not at the point of changing the house to LiFePO4, but when I do, my preliminary research indicates that I can continue using the AGM profile, at perhaps some small (<10%) sacrifice in LiFePO4 maximum charge. That's because the AGM float point may be below the minimum voltage a lithium's on-board battery management may want to see. And charging via the alternator will likely provide full charge to the lithiums.

Unlike AGMs, it looks like the LiFePO4 (with on-board BMS) is quite forgiving of charge profile discrepancies, with no shortening of battery life.

Using the VSR allows easy coupling and disconnecting of the two banks for jump starting, or charging the chassis from the house power source. So my hope is that there will be no need for hardware changes (outside of house battery type)

Yes. This would work for the house batteries – with some assumptions: 1. Your house batteries are AGM, or some other lead-acid variant. 2. You can get to the the batteries – or an appropriate set of terminals – to hook up the clamps. 3. The solar panel is appropriately sized for the parasitic draws on your house batteries.

Basically, this is a smaller version of what is installed on rigs with a “solar package”. So it should work for any lead-acid battery.

In an emergency I’ve thought of Gerry-rigging a means of depressing the on-dash button that taps the coach batteries to start the engine in the event of a dead chassis battery. I’m too ignorant to think of why such a set-up wouldn’t suffice as a means to keep the chassis tipped-off using the roof ZAMP, but there must be a myriad of reasons, one being the controller is likely specific for the OE AGM coach batteries, not the far lessor capacity chassis. I could imagine overcharging…overheating…and a fire. I have two additional 80w ZAMP added to the OE 100w in our early ’16 ‘K’.

The dash switch for the battery boost can be wired in a variety of ways. In a Travato, it’s hooked in through the relays in the “black box” by your rear axle. I’d need to look at a wiring diagram to figure out if it would work. They might only be connected when the ignition switch is “on”, for example. The main problem I can foresee with that is that if you had a bad cell in either set of batteries, then keeping them connected without some device to manage the connection might actually run down *both* sets of batteries, and then you’re sunk. That switch will work to start the battery though, and then you could let the engine run to charge it.

This charging solution requires your rig to be outdoors, or have the solar panel wired to the outdoors. For those of us storing indoors, and where power is available, a Battery Tender Jr. is a great solution.

You could really easily modify that cigarette lighter contraption to hook directly up to the battery. Just clip the connector off and add some alligator clips. If you wanted to get really fancy, put a fuse on the positive wire. No reason to throw away a perfectly good solar panel.

Yeah, you’re right, I could have done that. I had previously tested (years ago) the output from that panel and found the voltage to be crazy variable. I don’t know if it had just gone bad, or if it was unregulated, or what. I think the proper solar charging solution is a more stable solution.

I carry a small lithium jump starter that I’ve had to use more often then I’d like to admit. However, you do need to make sure it’s charged up before you can use it. The new gizmo out now is a jump charger that has a bank of big capacitors. You hook it up to your “Dead Battery” and it builds up a charge off of the residual energy in the battery, then you can crank your engine. I’m waiting for one to come out with enough power to crank a big V10 engine.

Quick question: why is the built in zamp controller not also wired to trickle charge the chassis battery ? Can the Winnebago installed one only deal with 1 battery ? It would be more elegant and convienient to use the already installed solar panel, charger, wiring etc. if that were possible…Alternatively, if a dual battery controller like this one from Zamp https://bendtrailers.com/zamp-30-amp-5-stage-dual-battery-deluxe-digital-pwm-zs-30ad/ ~$200 is required, how much modding skill would that take to wire up you think ? As always, thanks for your detailed experiments, mods, and informative and fun videos !

Good questions! Let me see if I can explain. First – this would work on any vehicle – even the family car with no solar on board. So there’s that. But yes, it would be a more elegant and fuss-free solution to integrate it with the solar that’s already on board. There are several devices that can handle this. One that comes to mind is the Trik-l-start. And then there are others like the Xantrex Echo Charge. These devices all work fairly well, but in our rig, we had an issue: We have a lithium house battery and an AGM chassis battery. These devices are all programmed to shut off at a certain voltage (so that they only charge the secondary battery when the first is actively charging). But that voltage was too low for our lithium battery. Our battery rests at over 13 volts – so these devices would never have shut off! So I didn’t go any further with those. As you point out, there is a Zamp product that will charge two battery banks. I actually called Zamp to inquire about it. At the time I called, the unit had not been vetted out by our battery manufacturer. So again, I didn’t go down that road too much further. Either of those two solutions would work if you had AGM batteries for both house and chassis, so for many folks, that may be a great solution. Just didn’t work for us. So I figured the next best thing would be to have completely independent systems, which is how I got to what you see in the video. As far as how much skill would be required – it would depend on what sort of access you had to your chassis and house batteries. I’d give it about a 5 on a scale of 1-10, just because you have to know where and how to add fuses. For us, it would have been incredibly simple (were it not for the battery chemistry mismatch). Like I said – great questions!!

I am new at this. I need to know much watt solar panel I need for the following. Charge my travel trailer battery, 12 volt. and a attached inverter 500 watts. I would like to run my computer, and charge the phone, and may be run the tv or and PS3 game console.